The effect of membrane fatty acid saturation on the activity of rat liver microsomal acyl coenzyme A cholesterol acyltransferase (ACAT) activity will be determined. One method for modifying microsomal fatty acid composition will be to feed the rats diets that are enriched in either saturated or polyunsaturated fat. Another procedure will be to prepare isolated hepatocyes and modify their membrane fatty acid composition in short term culture. The ACAT system will be solubilized from fat liver microsomes and the soluble system will be incorporated into phospholipid vesicles. This will provide a more controlled approach for modifying the lipid composition of the membrane in which ACAT is imbedded. Phospholipid exchange protein and phospholipid vesicles will be used in another approach to modify the lipid composition of the microsomes themselves. Taken together these three approaches should provide a comprehensive understanding of possible effects of membrane lipid composition on the activity of this important intracellular cholesterol metabolizing enzyme. Finally, additional hepatic enzymes that have been shown in preliminary studies to also be influenced by membrane lipid composition will be examined in some detail. These include five prime nucleotidase, sodium potassium ATPase and UDP glucuronyl transferase.